Elements, Compounds and Mixtures

Element:

• An element is a pure substance that contains only one kind of atom.
• All atoms in an element have the same atomic number (same number of protons).
• Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Carbon (C).
• Cannot be broken down into simpler substances by chemical reactions.
• Exists as atoms (e.g. He, Ne) or molecules (e.g. O₂, N₂) depending on the element.

Compound:

• A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio.
• Atoms are bonded together through chemical bonds (ionic or covalent).
• The properties of a compound are very different from the individual elements it is made from.
• Can only be separated into their elements by chemical reactions.
• Examples:
  • Water (H₂O): Made of hydrogen and oxygen atoms.
  • Sodium chloride (NaCl): Made of sodium and chlorine atoms.

Mixture:

• A mixture contains two or more substances (elements or compounds) that are physically combined but not chemically bonded.
• The components can be present in any proportion and retain their individual properties.

Separation of Mixtures by Physical Methods

Mixtures can be separated using physical methods because the substances in a mixture are not chemically bonded. The method used depends on the type of mixture and the properties of its components.

 Filtration

• Used to separate: Insoluble solids from liquids
• How it works: The mixture is poured through filter paper. The solid particles remain on the paper (residue) and the liquid passes through (filtrate).
• Example: Separating sand from water

 Evaporation

• Used to separate: A soluble solid from a solution
• How it works: The solution is heated until the liquid evaporates, leaving the solid behind.
• Note: The dissolved solid must not decompose on heating.
• Example: Obtaining salt from saltwater

 Distillation

• Used to separate: A solvent from a solution or liquids with different boiling points
• How it works: The mixture is heated so that the component with the lower boiling point evaporates first, is then cooled in a condenser, and collected separately.
• Types:
  • Simple Distillation: Used when separating a liquid from a dissolved solid (e.g. pure water from seawater)
  • Fractional Distillation: Used for separating two or more miscible liquids with different boiling points (e.g. ethanol and water)

 Chromatography

• Used to separate: Components of a mixture of dissolved substances (e.g. inks, dyes, pigments)
• How it works: A small sample is placed on filter paper and a solvent moves through the paper, carrying the components at different rates depending on their solubility and interaction with the paper.
• Key term: R_f value (Retention factor):
  \( R_f = \frac{\text{Distance moved by substance}}{\text{Distance moved by solvent}} \)
• Example: Identifying colouring agents in food dyes or ink